This invention relates generally to fluorescent lamps utilizing a dual phosphor layer of different phosphors to produce a high lumen output together with good color rendition. More particularly, this invention deals with such dual phosphor layer fluorescent lamps providing these desired results wherein the base layer of a modified calcium halophosphate phosphor is deposited on the glass surface at considerably lower dry coating weights as a means to reduce the overall phosphor weight in such lamps.
Efficient illumination of a white color along with good color rendition is now provided in fluorescent lamps with dual layer phosphor coatings employing various combinations of three narrow band emitting phosphor materials in the top phosphor layer each producing red, blue or green color emission. For example, there is disclosed in U.S. Pat. No. 4,088,923 a combination of said top tri-phosphor materials in the top layer of a dual phosphor layer fluorescent lamp which is superposed on a base layer of conventional calcium halophosphate phosphor to produce efficient visible illumination of a white color at a lower cost than provided with a single layer of the blended narrow bandwith emitting phosphors. A disclosure of suitable narrow band emitting phosphor materials for use in this manner is found in U.S. Pat. No. 3,937,998 wherein the blue color emission phosphor exhibits an emission band in the wavelength region from about 430 nanometers wavelength up to about 490 nanometers wavelength, the red color emission phosphor exhibiting an emission band in the wavelength range from about 590 nanometers wavelength up to about 630 nanometers wavelength, and with the green color emission phosphor producing emission extending from 520 nanometers wavelength up to about 565 nanometers wavelength. A dual layer fluorescent lamp phosphor coating of this type is also disclosed in U.S. Pat. No. 4,431,941 wherein reduced coating weights of the top phosphor layer having the same blend of these tri-phosphors are employed to still further reduce the overall phosphor costs of lamp manufacture. In said latter dual phosphor layer coating, however, a base layer coating weight of 4.4 milligrams per square centimeter is reported for the cool white type calcium halophosphate phosphor employed.
Since the overall phosphor costs in a fluorescent lamp still remain appreciable it is desireable to further decrease the overall amount of phosphor needed in the dual layer phosphor coating. As disclosed in the above referenced concurrently filed patent application, it is now possible to lower the dry coating weight for a single layer coating of a modified calcium halophosphate phosphor to a range from about 1.7 milligrams per square centimeter up to about 4.0 milligrams per square centimeter. While said improvement essentially maintains the lumen output and color rendering capability of a fluorescent lamp utilizing a single layer of the calcium halophosphate phosphor, however, it still lacks the higher CRI values of 68 or greater that are now achieved in these lamps with conventional dual layer phosphor coatings. Accordingly, it still remains desireable to further lower the total weight of said dual layer phosphor coatings while not sacrificing either lumen output or CRI values in such modified fluorescent lamps.